This study is designed to evaluate the intra-and interindividual differences in cytochrome P450 activity in HIV infected individuals. Using dextromethorphan, caffeine, and midazolam as probe drugs, cytochrome P450 activity will be quantified to determine intra- and interindividual variability in drug metabolizing enzyme activity. This will provide much needed baseline activity data on whch sample size calculation for further studies examining important influences on cytochrome P450 activity (i.e., cytokines) can be based.